Reinforcement of superconducting coils by high-strength materials

ABSTRACT

A reinforced superconducting coil and method for the reinforcement of such coil utilizing composite superconducting wires clad with high-strength material are disclosed.

This application claims the benefit of No. 60/264,861 filed Jan. 30, 2001.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The present invention resides in the field of superconducting coils and more particularly relates to the reinforcement of adiabatic superconducting coil magnets using composite superconducting wires that are clad with high-strength materials.

2. History of the Prior Art

Superconductivity is the phenomenon where, when a particular material, such as a wire, is subjected to successively colder temperatures, it undergoes a state transition where all electrical resistance disappears, i.e. the material can conduct electricity without generating any heat. At this stage the material is said to have become a superconductor. This phenomenon only occurs for specific metals/alloys and compounds. The state of superconductivity for a given superconductor is a function of its temperature, background magnetic field, and transport electric current.

Superconducting magnets almost exclusively consist of windings of superconducting wire, also known as coils. In very large magnets the “wire” may take the form of cabled superconducting wires. For some superconducting magnets the cable of wires is contained, together with helium for cooling, in a metal conduit. All wires used in building such magnets include insulation that surrounds the wire to be wound.

Turns and layers of the winding of a magnet are often bonded to each other by a bonding agent, such as epoxy. Such windings are said to be impregnated.

In high field magnets used for nuclear magnetic resonance (NMR) and for magnetic resonance imaging (MRI) the windings are usually solenoids. Often NMR and MRI magnets are comprised of a number of solenoid coils that are electrically connected. The magnetic fields produced by the system of coils combine to produce an intended magnetic field intensity over an intended volume inside the system of coils of the magnet. The field intensity of a solenoid, as well as most other magnet configurations, scales with the number of turns of the wire that makes up the windings of the coil and the electric current (amperes) that passes through the wire. Often it is said that the field intensity scales with “ampere-turns.” For reasons of: 1) minimizing space, 2) using less superconducting wire, and more importantly 3) reducing the stored energy of the coil, most coils used in NMR and MRI magnets are designed and built such that a given coil is as compact as possible.

When current passes through the wire of a coil, a magnetic field is produced. When current passes through a system of coils, a combined magnetic field is produced. The magnetic field (B) which is produced by a coil or a system of coils around a given turn inside the winding volume (pack) of a coil interacts with the current in the wire and produces a force on the wire, referred to as the Lorentz Force. Because the orientation of the magnetic field varies according to the location of the turn within the coil, or system of coils, the direction of the force varies also. The Lorentz Forces are balanced by the wire and other materials that make up the coil, resulting in mechanical stresses in the wire. In a given solenoid coil the Lorentz Forces on the turns combine to produce two components of stress in the wire. A tensile hoop stress is developed in the wire of magnitude B_(x)I_(x)R where B is the local field strength, I is the current and R is the radius of curvature. A transverse stress is also incident because radial components of the field in regions of a winding produce an axial force. Of these components of stress the hoop stress is generally the more significant, but both must be considered in the structural design of a high field magnet. When a solenoid coil is charged by current passing through it, there is a balance between its electromagnetic energy and mechanical elastic energy. Therefore a charged coil, or a system of coils, stores energy.

Design parameters and conventional approaches to solenoid design are discussed, for example, in: Superconducting Magnets, M. N. Wilson, Oxford University Press, New York, N.Y. (1983) and Case Studies in Superconducting Magnets, Y. Iwasa, Plenum Press, New York, N.Y. (1994).

The stresses on the wires inside MRI and NMR coils may reach hundreds of MPa. Therefore a main part of the design optimization of coils is calculation of stresses and bearing of the stresses by the coil.

To achieve the highest possible fields inside the bore of the NMR and MRI magnets, and most effectively support against Lorentz Forces, the wires used in these magnets are usually a solid monolithic wire, and their winding packs are devoid of any space for helium which is sometimes used as a heat sink inside the winding pack of some superconducting magnets. The coils of NMR and MRI magnets are comprised of solid matter. These are often referred to as being adiabatic coils as any release of local energy within the winding pack is absorbed by the winding pack itself first, and then it is transferred to the cooling medium that is external to the winding pack.

For reasons that relate to stability of superconductors, superconducting wires used in most magnet application are multifilamentary (MF) composites. [see for example Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995]. The superconducting filaments in MF wires in use in most superconducting magnets today are made from niobium-titanium (Nb—Ti) alloy. The Tc and Bc₂ for Nb—Ti are about 10 Kelvin (K) and about 10 (T), respectively. Nb—Ti alloy is ductile and basically insensitive to strain; and its use in fabricating MF wires, and subsequently in a magnet, is straightforward and comparatively less expensive than using other materials. MF Nb—Ti composite wires are often comprised of Nb—Ti filaments inside a copper or copper alloy matrix.

Superconducting magnets for operation at fields higher than about 10 T rely principally on the use of type A15 superconductors. Among the A15 superconductors the Nb₃Sn based wires are most practical for large-scale production, basically due to the fact that fabrication of Nb₃Sn is economical and less complicated. Almost all operating A15 magnets to date have used Nb₃Sn conductors. The Tc and Bc₂ for Nb₃Sn are about 18 Kelvin (K) and about 23 (T), respectively. Other A15 superconductors, such as Nb₃Al, that have better superconducting properties than Nb₃Sn are under development.

Nb₃Sn, like other A15 phases, is an intermetallic compound and is inherently brittle. Therefore it does not lend itself to normal conductor fabrication methods where a given material undergoes significant plastic deformation. For most applications in magnet technology, Nb₃Sn superconductors are produced by a two-step process in which a multifilamentary composite wire, that contains Nb and Sn in separate regions, is formed into wire (Nb₃Sn precursor wire) and then, during a subsequent reaction heat treatment at, for example 650 C.-750 C., the Nb₃Sn is formed by solid state reaction. MF Nb₃Sn composite wires are often comprised of Nb₃Sn filaments inside a copper and bronze alloy matrix. Because Nb₃Sn is brittle, windings of Nb₃Sn wire are often produced by winding un-reacted wires and then heat-treating the winding as a whole. This approach is referred to as the wind-then-react method. If the winding diameter is large enough the winding can be performed using a reacted Nb₃Sn wire. This approach is referred to as react-then-wind method. Because MF Nb₃Sn wires have to be heat-treated at temperatures of 650 C.-750 C., the non-superconducting matrix in the wires which is most often copper or bronze is in a fully annealed state and is therefore mechanically weak and cannot contribute much to the bearing of Lorentz Forces. Therefore it is particularly desirable to reinforce Nb₃Sn wires and coils for NMR and MRI magnet applications.

A main challenge of design and manufacturing coils for NMR and MRI magnets is to minimize the size of the coils by using conductors that have enough superconducting material to provide the required ampere-turns and optimizing/minimizing structural material to bear the mechanical stresses.

Up to the present time the following methods have been used to reinforce the windings of large diameter or high field solenoids:

a) extra cold worked (hard) copper has been incorporated into the wire wherein the non-superconducting part of a wire that carries no current is increased and therefore the ampere-turns for a given winding pack is decreased;

b) winding sections have been over-banded with a strong material, including steel, beryllium copper, carbon fiber, etc.; and

c) attempts have been made to incorporate reinforcements in addition to the use of copper and copper alloys inside superconducting wires.

One disadvantage to the use of hard copper as the reinforcement is that the strength of hard copper is limited to about 40,000 psi (275 MPa) in consequence of which in many magnet windings more copper is needed for reinforcement than for stabilization.

The result is a lower overall winding current density than could be obtained with the separation of functions, that is, enough copper for stabilization or quench protection and enough high-strength reinforcement for stress containment. A disadvantage of overbanding is that high radial strain in the active winding is needed to transfer the electromagnetic loads to the overbanding. The most efficient and compact winding is obtained when the reinforcement is distributed throughout the active winding. Variations in distributing reinforcement throughout the active winding, though, can create disadvantages inside a superconducting wire in that it complicates the wire manufacturing process for both Nb—Ti and Nb₃Sn MF wires because the reinforcing material needs to be both mechanically compatible with the wire manufacturing process which limits the choice of materials as such materials must also be chemically inert to the chemistry of the superconducting wires.

SUMMARY OF THE INVENTION

It is an object of this invention to create adiabatic coil windings having great resistance to the aforementioned stresses. In this invention a reinforcement is incorporated directly on a MF superconducting composite wire as cladding. This is done after MF superconducting wire fabrication is nearly completed. It is assumed that insulation is selected and applied appropriately on the superconducting wires. In this way, when the reinforced wire is wound into a coil, the structural support is well distributed throughout the winding.

The prior art does not teach this approach because adiabatic magnet designs of the past have required copper or copper alloys to form the exterior of a superconducting wire for what has been commonly perceived as optimum stability and protection. This invention does not share this design philosophy.

The present invention contemplates that adiabatic superconducting magnets are prone to instability arising from thermal transients within the winding. For example, when the bonding agent used in the winding pack of a coil cracks as a result of straining associated with cooling, motion against fixtures, or charging of the magnets, heat arises in the winding pack, producing a temperature pulse that diffuses into the superconducting material in the core of wire and may raise the temperature locally above the current sharing temperature. The result may be a premature loss of superconductivity, referred to as quench. For a given heat pulse, the temperature rise is a function of the heat conductivity and heat capacity of the material affected by the heat pulse. In contrast to conventional NMR and MRI winding packs that use copper or copper alloys on the surface of the wires, in this invention the wires are clad by materials of low thermal conductivity, such as steel alloy, therefore the diffusion of heat into the core of wire will be impeded and heat diffused to a wider region affecting a relatively larger mass. For example, a steel-clad wire is more resistant to thermal transient than a wire with copper at the outer surface. Furthermore, steel being tough and rigid, protects the wire from abuse more efficiently than prior art outer surface materials, such as copper.

Cladding with high-strength material is particularly apposite for a Nb₃Sn wire which has been reacted before winding. Indeed, the clad Nb₃Sn wire may make “react-then-wind” magnets much more practical than at present. It should be understood that Nb₃Sn is discussed only as an example of A15 superconductor and the principals of invention apply to all MF A15 wires.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view through a multifilamentary wire of the prior art.

FIG. 2 illustrates a cross-sectional view through a reinforced wire of this invention in which steel takes the form of a sheath around the outer surface of the superconductor-copper composite wire.

FIG. 3 illustrates a cross-sectional view through a prior art conductor configuration with a known superconducting current carrying capacity and mechanical strengths.

FIG. 4 illustrates a cross-sectional view of the steel-clad superconducting wire of this invention of equivalent strength to the prior art wire of FIG. 3.

FIG. 5 illustrates a cross-sectional view of another embodiment of the steel-clad superconducting wire of this invention having a different cross-sectional shape.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 illustrates a cross-sectional view of prior art composite wires 10 made of filamentary Nb—Ti or Nb₃Sn within a copper or copper alloy matrix 12. FIG. 2 illustrates a cross-sectional view of a similar wire to that of FIG. 1 but surrounded with stainless steel reinforcing material 14. The steel takes the form of a sheath over the composite wire. This arrangement differs from that of the so-called cable-in-conduit superconductor (CiCC) in that no void space unfilled with metal is left in the cross section. Where the superconductor is composed of Nb—Ti, manufacturing processes can include the following: Nb—Ti-copper composite wire of a given diameter and length is clad by a given layer of steel by folding and welding a continuous sheet of the steel. The steel is then drawn over the Nb—Ti-copper wire to provide intimate contact and achieve a final target diameter and geometry. For Nb₃Sn wire the cladding procedure is the same as above but the steel cladding is put on the Nb₃Sn precursor wire before heat treatment. Therefore in the steel selection one should be mindful of the effect of heat treatment on the properties of the particular steel.

A stable adiabatic magnet must have no sources of temperature excursion greater than the temperature margin of operation. It is known that temperature perturbations arise from cracking of epoxy impregnation or frictional movement between a winding and its surrounding form. Temperature rise may also result from the decay of induced shielding currents, but that is a negligible effect during normal charge. The elimination or at least the minimization of these triggers is essential in the design and construction of the magnet. The specification of the temperature margin has hitherto been based principally on design experience. The quantitative specification of temperature margin which usually is specified as the margin between operating current and critical current is complex, because the perturbations that may cause temperature excursions are ill defined. However, in the present invention the use of a generous amount of a high residual resistivity ratio (RRR) normal conductor, e.g. copper, is not essential for stability, and furthermore, it is not essential or even desirable, for quench protection. For example, in the case where a central core containing the superconducting filaments is surrounded by a low RRR copper sheath, which in turn is surrounded by a high-strength reinforcement and where a frictional or mechanical perturbation occurs at the outer surface of this combination, the high-strength sheath acts to provide high thermal resistance. The heat generated at its outer surface diffuses slowly inwards and meets the low RRR copper annulus. This conductor has high conductivity so that the heat issuing from the reinforcement diffuses axially as well as radially. This heat diffusion results in a much attenuated heat pulse entering the core. Although the copper sheath provides little longitudinal electrical conductivity, because its RRR is low, its relative conductivity is still high when compared with the matrix of the core. Thus, this combination will provide almost as good a stabilizing effect as a high RRR sheath. It is important to note that, with one exception, all perturbations produce heat pulses which diffuse inwards from the outside of the conductor, not from the core outwards. The exception is a flux jump caused by the collapse of shielding currents, but such an event is rare in a wire of appropriate twist pitch. Thus, if a perturbation occurs which raises the temperature of the superconducting filaments to just below the current sharing temperature, no current flows in the copper. If, however, the temperature exceeds the current sharing temperature, then current will flow in the copper. Whether the superconductor recovers from the subsequent transient then depends largely on the thermal conductivity of the copper, because longitudinal heat transport is then the only mechanism that can remove heat faster than it is generated. Thus, a composite conductor that includes both (1) a high-strength, high-resistivity cladding and (2) copper positioned in between the cladding and the superconducting filaments is preferred for optimized adiabatic superconducting coils.

Essential to the composition of the conductor is a sufficient normal conductor to provide the necessary quench protection. A combination of structure and stabilizer to be incorporated into the conductor could be, for example, stainless steel 316LN that has high elastic modulus, 200 GPa and high yield strength (up to 1.6 GPa for fully hard wires) and therefore much less of it is required to provide the same structural support as hard copper (elastic modulus of 130 GPa and strength of 0.4 Gpa). As an example of the use of the composite superconducting wire of this invention, first consider the prior art conductor that was used in the largest Nb—Ti coils of the 600 MHz magnet designed and built by J. Williams [J. E. C. Williams, S. Pourrahimi, Y. Iwasa, and L. J Neuringer, “A 600 MHz Spectrometer Magnet,” IEEE Trans. Vol. Mag-25, pp. 1767-1770, 1989]. The cross-section of this conductor is depicted in FIG. 3. The copper/superconductor ratio is 4/1. As an alternative the present invention provides the conductor shown in FIG. 4. This conductor has a core with a superconductor/copper ratio of 1/1 which has a stainless steel 316LN cladding 14. The conductors shown in FIGS. 3 and 4 have equivalent mechanical properties and superconducting critical properties, that is, they can carry the same current at various fields, but the overall area of the conductor of this invention is about half that of the prior art conductor illustrated in FIG. 3. Thus, by using the conductor of this invention the same ampere/turns can be achieved in a winding pack that is half as thick and weighs about half as much as the prior art conductor.

The use of superconducting wires having a square, or low aspect ratio rectangular, cross section has become favored for fabrication of high performance NMR and MRI magnets. The conductor configuration illustrated in FIG. 5 has an equivalent cross-sectional area as that of the conductor illustrated in FIG. 4 but can achieve a higher overall critical current density for a given winding pack thickness, and is mechanically more robust.

Although the present invention has been described with reference to particular embodiments, it will be apparent to those skilled in the art that variations and modifications can be substituted therefor without departing from the principles and spirit of the invention. 

We claim:
 1. A superconductive coil comprising: coils of superconductive composite wire; and a cladding surrounding said wire wherein said cladding is made of high-strength reinforcing material of high resistance and low thermal conductivity.
 2. A superconductive coil, comprising: a composite superconductive wire; and a cladding surrounding said wire without voids, said cladding made of a high-strength, high-resistance reinforcing material of low thermal conductivity.
 3. The structure of claim 2 wherein said composite wire is selected from the group of NbTi-copper, Nb₃Sn-copper, niobium-bronze-copper and any A15 superconductor filaments-copper and said reinforcing material is selected from the group of steel alloy, nickel alloy, superalloy and stainless steel alloy.
 4. The structure of claim 3 wherein said superconductive wire has a circular cross section.
 5. The structure of claim 4 wherein said superconductive wire has a rectangular cross section.
 6. A method of manufacture of a superconducting coil, comprising the steps of: providing a superconducting composite wire; surrounding said wire without forming voids with a cladding of high strength, high-resistance cladding of low thermal conductivity; and forming said wire in said coil.
 7. The method of claim 6 further including the steps of: forming said composite wire of material selected from the group of NbTi-copper, Nb₃Sn-copper and any A15 superconductor filaments-copper; and forming said cladding of a material selected from the group of steel alloy, nickel alloy, superalloy and stainless steel alloy.
 8. The method of claim 7 further including the step of forming said wire into a shape having a circular cross-section.
 9. The method of claim 7 further including the step of forming said wire into a shape having a rectangular cross-section. 